The invention concerns a method for controlling the parallel operation of multiple d.c. voltage converters having the features indicated in the preamble of Claim 1, as well as a device for controlling the parallel operation of multiple d.c. voltage converters having the features indicated in the preamble of Claim 7.
To improve the performance of d.c. voltage converters, multiple d.c. voltage converters can be connected in parallel and used in the sense of a master/slave operation. The d.c. voltage converter serving as master thereby takes over the voltage regulation in the total output-side system. The lower-order converter or converters, as current sources controlled by the master, help increase the output performance. In traditional systems, an additional cable connection is required to transfer the control commands between master and slave, by way of which analog, time-continuous current setpoints are transmitted to the slave converter or slave converters.
DE 195 46 495 A1 has already made known a circuit configuration and a method for an even distribution of electrical energy. Multiple static power converters are thereby operated as power supply units with rectifiers for supplying a common d.c. load. The power supply units are connected to each other by way of a bus system. When the system is operated, current is automatically distributed evenly.
Moreover, DE 198 05 926 A1 makes known a device and a method for the controlled parallel operation of d.c. voltage converters, in particular in a multi-voltage electric system of a vehicle. One of the converters thereby works in the active area, and the others work either in full-load operation or no-load operation.
Moreover, patent application DE 199 33 039 makes known a d.c. voltage converter, the control signal of which is created using a voltage regulator and a current regulator. A limiter is provided between the voltage regulator and the current regulator, which serves to limit the output signal of the voltage regulator. By specifying suitable values for a voltage setpoint and a limit value signal for the current setpoint, various operating modes can be achieved. These include a mode with controlled output voltage and a mode with controlled output current. The latter is particularly suited for a parallel operation of multiple d.c. voltage converters, whereby a higher-order d.c. voltage converter takes over the voltage regulation and only assigns the setpoint for the output current to be set to the other converters. The lower-order converters therefore work as current source.
The advantages of a method according to the invention and a device according to the invention are, in particular, that, due to the control strategy employed, the current distribution between the converters is optimized in terms of a control reserve that is as large as possible. Since the d.c. voltage converter working with voltage regulation and performing a master function is usually operated at its half nominal current, its half nominal current is available to it for reacting dynamically to short-run load variations in both the positive and negative directions.
This is an advantage in particular when a digital, time-discrete information transfer takes place between the d.c. voltage converters. Such an information transfer is subject to delays, so that the lower-order d.c. voltage converters working as current source are not immediately available. This can lead to control differences. Such control differences can be corrected rapidly by the d.c. voltage converter performing a master function, which has access to sufficient control reserves for short-run load variations in the positive direction as well as in the negative direction.